The present invention relates to an objective lens actuator of an optical disk device for optically writing or reading information on a disklike recording medium.
In a known optical disk device, an objective lens is driven in two directions, i.e. along a vertical optical axis perpendicular to a face of a disklike recording medium (hereinbelow, referred to as a "focusing direction F") and along a radial direction parallel to the face of the recording medium (hereinbelow, referred to as a "tracking direction T") so as to correct focusing error resulting from vertical motion of the recording medium due to its warpage and tracking error due to radial deviations of the disc, etc. such that recording and reproduction are performed optically.
In recent years, trends towards portable use and personal use of a compact disk player or a data file device employing an optical disk are gaining momentum and thus, an optical pickup for use in the compact disk player or the data file device is also required to be made more compact, especially less thick.
FIGS. 1 to 3 show one example of an known objective lens actuator of the optical pickup. The known objective lens actuator includes an objective lens 1, a lens holder 2 for holding the objective lens 1, a pair of permanent magnets 3a and 3b, wires 4a to 4d for supporting the lens holder 2 and a pair of magnetic yokes 5a and 5b. A magnetic circuit is formed by the permanent magnets 3a and 3b and the magnetic yokes 5a and 5b.
The known objective lens actuator further includes a tracking coil 6 wound vertically around the lens holder 2, a focusing coil 7 wound horizontally around the lens holder 2, a stationary member 8, a base 9 and a reflective mirror 10 for guiding to the objective lens 1 a light beam emitted from a laser diode (not shown) acting as a light source. One end of each of the wires 4a to 4d is attached to the stationary member 8, while the other end of each of the wires 4a to 4d is secured to the lens holder 2 such that the wires 4a to 4d extend parallel to each other. The permanent magnets 3a and 3b, the magnetic yokes 5a and 5b, the stationary member 8 and the reflective mirror 10 are fixed to the base 9. A movable member is constituted by the objective lens 1, the lens holder 2, the tracking coil 6 and the focusing coil 7.
The operation of the known objective lens actuator is described hereinbelow. In order to correct a focusing error resulting from vertical motion of the recording medium due to its warpage or tracking error due to radial deviations in the disc, etc., the objective lens 1 is driven in the focusing direction F and the tracking direction T as follows. The lens holder 2 carrying the objective lens 1 is supported by the wires 4a to 4d. A light beam emitted from the laser diode is guided by the reflective mirror 10 so as to be incident upon the objective lens 1.
In an electrokinetic type of focusing transducer, the focusing coil 7 is disposed in a gap of the magnetic circuit formed by the permanent magnets 3a and 3b and the magnetic yokes 5a and 5b. In order to drive the objective lens 1 in the focusing direction F, the lens holder 2 carrying the objective lens 1 is translated through the wires 4a to 4d by the focusing transducer.
Likewise, in an electrokinetic type tracking transducer, the tracking coil 6 is disposed in the gap of the magnetic circuit formed by the permanent magnets 3a and 3b and the magnetic yokes 5a and 5b. In order to drive the objective lens 1 in the tracking direction T, the lens holder 2 carrying the objective lens 1 is translated through the wires 4a to 4d by the tracking transducer. In addition, the known objective lens actuator as a whole is driven by a motor (not shown) in an access direction Ac on the recording medium. The access direction Ac is the same as the tracking direction T.
Meanwhile, the objective lens 1 is mounted on the lens holder 2 outside, in the tracking direction T, of an available magnetic field of the magnetic circuit formed by the permanent magnets 3a and 3b and the magnetic yokes 5a and 5b. Outside the available magnetic field of the magnetic circuit, the lens holder 2 has a thickness t1 in the focusing direction F at its portion where the objective lens 1 is mounted as shown in FIG. 2. On the other hand, in the available magnetic field of the magnetic circuit, the lens holder 2 has a thickness t2 in the focusing direction F such that the thickness t1 is smaller than the thickness t2. As a result, the objective lens 1 can be brought closer to the reflective mirror 10 and thus, the known objective lens actuator as a whole can have a relatively small thickness.
However, in the above-described objective lens actuator, the objective lens is disposed outside the available magnetic field of the magnetic circuit in order to make the optical head thinner. Therefore, the center of the driving force for driving the lens holder is offset from the center of the objective lens. The frequency response in the focusing direction is adversely affected by the deviation of the center of the driving force from center of gravity of the movable member.